The art of thermal printing has been established for many years. In general the principle employed is to operate a heating element to cause it to mark a sheet of heat-sensitive paper or to cause transfer of a mark from a transfer sheet to a paper sheet. Characters are formed from a combination of such marks, or in some cases by shaping the elements in character configuration.
Most of the devices in the art employ discrete elements, either in a line or a matrix formation. Examples of such devices are shown in U.S. Pat. Nos. 3,453,647 (Bernstein et al), 3,953,708 (Thornburg), 3,955,068 (Shaheen), 4,030,408 (Miwa), 4,039,065 (Seki et al), 4,136274 (Shibata et al), 4,203,119 (Naguib et al), 4,242,565 (Schoon), and 4,250,375 (Tsutsumi et al). As the requirements for high quality printing, by using smaller dot or mark sizes, goes up, these systems offer greater problems. Firstly, as each element has to be insulated from adjacent elements, the space required between elements becomes more significant and limits the number of elements per unit length or area. Secondly, the number of drive lines must be equal to at least the number of elements plus one, in the case in which there is a common earth drive to each element, and it may be as great as twice the number of elements. Though modern manufacturing techniques can produce extremely dense circuit patterns, it must be remembered that in the production of heat, relatively large currents are employed, so the cross-sectional area of the drive lines can not fall below minimum, this reduces the minimum density of circuit packaging and, therefore, limits the density of elements.
U.S. Pat. No. 4,099,046 (Boynton et al) shows an arrangement in which one of the disadvantages of the prior systems is avoided. In this arrangement a continuous bar of resistive material is employed. Connections are made to respective sides of the bar from connectors which are staggered on one side of the bar relative to the other side. Thus each printing element is defined by the resistive material between a connection on one side of the bar and an adjacent connection on the other side of the bar. The elements, therefore are defined in a zig-zag formation along the bar. Though this arrangement avoids the necessity of insulating the printing elements one from another, it does require the number of leads to be equal to the number of elements plus one. In addition, as the connections are made to the sides of the bar, in fact to small areas below the bar, their size is limited, which can cause problems when relatively high currents are involved.
It is, therefore, an object of the invention to provide a thermal print head in which the disadvantages of the prior print heads are substantially reduced.
It is a further object of the invention to provide a thermal print head in which the required number of connections are reduced as compared with prior print heads.
According to the invention, there is provided a thermal print head comprising first and second longitudinally extending, electrically resistive, continuous print bars positioned in parallel, a first and a second set of leads connected to the first and second bars respectively, each lead being individually connected to the associated bar at a position substantially equidistant from the connection positions of immediately adjacent leads, and a third set of leads of which each lead is individually connected to both bars at a position substantially midway between adjacent connections from leads of the first and second sets.